Residents across the province had received a message saying “emergency staff are responding to the situation,” at the Pickering nuclear power station, but were told they did not need to take protective measures at this time.
Authorities had also said that no abnormal radioactivity had been released.
The Ontario Power Generation tweeted, "Important update: the alert regarding #Pickering Nuclear was sent in error. There is no danger to the public or environment."
Local officials called for an investigation into how the terrifying false alarm had been sent out.
"Like many of you, I was very troubled to have received that emergency alert this morning. While I am relieved that there was no actual emergency, I am upset that an error such as this occurred," Pickering Mayor Dave Ryan said on Twitter.
"I have spoken to the province, and am demanding that a full investigation take place."
Toronto Mayor John Tory complained that many of the city's 3 million residents had been unnecessarily alarmed and also pushed for a probe, citing what he said were "far too many unanswered questions".
The plant – one of the largest in the world – started operating in 1971 and has a power-generating capacity of 3,100 megawatts when fully active.
Toronto has a population of around three million.
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